Method and apparatus for treating a hydrocarbon stream

ABSTRACT

The present invention relates to a method of treating a hydrocarbon stream such as a natural gas stream, the method at least comprising the steps of: (a) supplying a partially condensed feed stream ( 10 ) to a first gas/liquid separator ( 2 ), the feed stream ( 10 ) having a pressure &gt;50 bar; (b) separating the feed stream ( 10 ) in the first gas/liquid separator ( 2 ) into a first vaporous stream ( 20 ) and a first liquid stream ( 70 ); (c) expanding the first vaporous stream ( 20 ), thereby obtaining an at least partially condensed first vaporous stream ( 30 ); (d) supplying the at least partially condensed first vaporous stream ( 30 ) to a second gas/liquid separator (4); (e) separating the stream ( 30 ) as supplied in step (d) in the second gas/liquid separator ( 4 ) into a second vaporous stream ( 60 ) and a second liquid stream ( 40 ); (f) increasing the pressure of the second liquid stream ( 40 ) to a pressure of at least 50 bar, thereby obtaining a pressurized second liquid stream ( 50 ); and (g) returning the pressurized second liquid stream ( 50 ) to the first gas/liquid separator ( 2 ).

The present invention relates to a method of treating a hydrocarbonstream such as a natural gas stream, in particular in a process for theproduction of liquefied natural gas.

Several methods of treating a natural gas stream are known, e.g. toremove undesired components from the natural gas and/or to meet therequired specifications of a client.

Also, several methods of liquefying a natural gas stream therebyobtaining liquefied natural gas (LNG) are known. It is desirable toliquefy a natural gas stream for a number of reasons. As an example,natural gas can be stored and transported over long distances morereadily as a liquid than in gaseous form, because it occupies a smallervolume and does not need to be stored at high pressures.

Usually, the natural gas stream to be liquefied (mainly comprisingmethane) contains ethane, heavier hydrocarbons and possibly othercomponents that are to be removed to a certain extent before the naturalgas is liquefied. Also to this end, the natural gas stream is treated.One of the treatments may involve the removal of at least some of theethane, propane and higher hydrocarbons such as butane and propane.

A known method of treating a natural gas stream is disclosed in U.S.Pat. No. 5,291,736 relating to a method for the liquefaction of naturalgas, at the same time separating hydrocarbons heavier than methane.

As the treating process, whether or not forming part of a liquefactionprocess, is highly energy consuming there is a constant need to providealternative processes of treating natural gas, wherein the energyconsumption is reduced.

It is an object of the invention to meet the above need and to provide aprocess in which the energy consumption is reduced.

It is a further object of the present invention to provide analternative method for treating a natural gas stream.

One or more of the above or other objects are achieved according to thepresent invention by providing a method of treating a hydrocarbon streamsuch as a natural gas stream, the method at least comprising the stepsof:

-   -   (a) supplying a partially condensed feed stream to a first        gas/liquid separator, the feed stream having a pressure >(above)        50 bar;    -   (b) separating the feed stream in the first gas/liquid separator        into a first vaporous stream and a first liquid stream;    -   (c) expanding the first vaporous stream obtained in step (b),        thereby obtaining an at least partially condensed first vaporous        stream;    -   (d) supplying the at least partially condensed first vaporous        stream obtained in step (c) to a second gas/liquid separator;    -   (e) separating the stream as supplied in step (d) in the second        gas/liquid separator into a second vaporous stream and a second        liquid stream;    -   (f) increasing the pressure of the second liquid stream obtained        in step (e) to a pressure of at least 50 bar, thereby obtaining        a pressurized second liquid stream; and    -   (g) returning the pressurized second liquid stream (50) obtained        in step (f) to the first gas/liquid separator.

In an alternative embodiment, the invention relates to a method oftreating a hydrocarbon stream such as a natural gas stream, the methodat least comprising the steps of:

-   -   (a) supplying a partially condensed feed stream (10) to a first        gas/liquid separator (2), the feed stream (10) preferably having        a pressure >30 bar;    -   (b) separating the feed stream (10) in the first gas/liquid        separator (2) into a first vaporous stream (20) and a first        liquid stream (70);    -   (c) expanding the first vaporous stream (20) obtained in step        (b), thereby obtaining an at least partially condensed first        vaporous stream (30);    -   (d) supplying the at least partially condensed first vaporous        stream (30) obtained in step (c) to a second gas/liquid        separator (4);    -   (e) separating the stream (30) as supplied in step (d) in the        second gas/liquid separator (4) into a second vaporous stream        (60) and a second liquid stream (40);    -   (f) increasing the pressure of the second liquid stream (40)        obtained in step (e) to a pressure of at least 30 bar, thereby        obtaining a pressurized second liquid stream (50); and    -   (g) returning the pressurized second liquid stream (50) obtained        in step (f) to the first gas/liquid separator (2).

It has surprisingly been found that using the method according to thepresent invention, a significant reduction of energy consumption may beobtained. The method according to the invention is especiallyadvantageous as the feed stream is available at a relatively highpressure, typically >(above) 50 bar, preferably above 55 bar, morepreferably above 60 bar.

Whenever in the specification and claims reference is made to a pressurein bar, this is a pressure in bar (absolute).

According to the present invention no expensive refrigerant scheme hasto be used to cool the first vaporous stream.

The hydrocarbon stream may be any suitable stream to be treated, but isusually a natural gas stream obtained from natural gas or petroleumreservoirs. As an alternative the natural gas stream may also beobtained from another source, also including a synthetic source such asa Fischer-Tropsch process.

Usually the natural gas stream is comprised substantially of methane.Preferably the feed stream comprises at least 60 mol % methane, morepreferably at least 75 mol %, such as at least 80 mol % methane.

Depending on the source, the natural gas may contain varying amounts ofhydrocarbons heavier than methane such as ethane, propane, butanes andpentanes as well as some aromatic hydrocarbons. The natural gas streammay also contain non-hydrocarbons such as H₂O, mercury, N₂, CO₂, H₂S andother sulphur compounds.

If desired, the feed stream containing the natural gas may bepre-treated before feeding it to the first gas/liquid separator. Thispre-treatment may comprise removal of undesired components such as H₂O,mercury, N₂, CO₂, H₂S and other sulphur compounds, or other steps suchas pre-cooling or pre-pressurizing. As these steps are well known to theperson skilled in the art, they are not further discussed here.

Usually the feed stream has a temperature in the range from ambient to90° C., preferably from 20° C. to 80° C. Preferably the pressure of thefeedstream is in the range from more than 50 bar to 100 bar, morepreferably from more than 55 bar to 90 bar, even more preferably frommore than 60 bar to 80 bar.

The first and second gas/liquid separators may be any suitable means forobtaining a vaporous stream and a liquid stream, such as a vessel, ascrubber, a distillation column, etc. Usually the first gas/liquidseparator comprises a column having 1-30 trays, preferably 1-15 trays.In the embodiment of the invention described with reference to FIG. 1,the second gas/liquid separator usually comprises a simple vessel withonly one tray. In the embodiment of the invention described withreference to FIG. 2, the second gas/liquid separator preferablycomprises a column having 1-30 trays, more preferably 1-15 trays.

Alternatively the first and second gas/liquid separators may each beprovided with packing (random or structured). When the gas/liquidseparator is provided with trays, a distillation stage corresponds toone tray, and when the gas/liquid separator is provided with packing(random or structured) a distillation stage corresponds to a theoreticalstage.

Where in the specification and in the claims a level of introducing astream into the gas/liquid separator is defined relative to introducinganother stream, there is at least one distillation stage between the twolevels, the same applies to defining the level of removing a stream fromthe gas/liquid separator. The top of the gas/liquid separator is thatpart of the gas/liquid separator that is located above the uppermostdistillation stage, and the bottom of the gas/liquid separator is thatpart of the gas/liquid separator that is located below the lowermostdistillation stage.

The first liquid stream and the second vaporous stream may be used asproduct streams or may be further processed, if desired.

In step (f) of the method of the present invention, the pressure of thesecond liquid stream obtained in step (e) is increased to a pressure ofat least 50 bar, thereby obtaining a pressurized second liquid stream.Preferably, the pressure of the second liquid stream is increased to apressure in the range from more than 50 bar to 100 bar, more preferablyfrom more than 55 bar to 90 bar, even more preferably from more than 60bar to 80 bar.

Typically, the pressure of the second liquid stream is in the range from0 to 5 bar higher than the pressure in the first gas/liquid separator,preferably from 0 to 2 bar higher, even more preferably from 0 to 1 barhigher, in particular substantially the same pressure.

It is preferred according to the present invention that in step (a) thefeed stream is supplied as at least two different streams to the firstgas/liquid separator, the feed stream comprising a higher feed streamand a lower feed stream. In this embodiment, the higher feed stream isfed at a warmer (i.e. higher) point of the first gas/liquid separatorthan the lower feed stream (that is fed at a lower, i.e. colder, pointof the first gas/liquid separator).

Further it is preferred that the higher feed stream is cooled,preferably against the second vaporous stream obtained in step (e). Tothis end a heat exchanger may be used.

Also it is preferred that the first liquid stream obtained in step (b)is supplied to a third gas/liquid separator thereby obtaining a thirdvaporous stream and a third liquid stream. Preferably the third vaporousstream is combined with the second vaporous stream.

In a further aspect the present invention relates to an apparatus fortreating a hydrocarbon stream such as a natural gas stream, theapparatus at least comprising:

-   -   a first gas/liquid separator for separating a partially        condensed feed stream into a first vaporous stream and a first        liquid stream;    -   an expander for expanding the first vaporous stream;    -   a second gas/liquid separator for separating the expanded first        vaporous stream into a second vaporous stream and a second        liquid stream; and    -   a pressurizing unit for increasing the pressure of the second        liquid stream to at least 50 bar before being returned to the        first gas/liquid separator.

Preferably the first gas/liquid separator comprises at least two inletsfor the feed stream, including an inlet for a higher feed stream and aninlet for a lower feed stream.

It is especially preferred that the apparatus further comprises a heatexchanger for cooling the higher feed stream against the second vaporousstream.

Further it is preferred that the apparatus further comprises a thirdgas/liquid separator for separating the first liquid stream into a thirdvaporous stream and a third liquid stream. Preferably the third vaporousstream can be combined with the second vaporous stream.

Hereinafter the invention will be further illustrated by the followingnon-limiting drawing. Herein shows:

FIG. 1 schematically a process scheme in accordance with an embodimentof the present invention; and

FIG. 2 schematically a process scheme in accordance with anotherembodiment of the present invention.

For the purpose of this description, a single reference number will beassigned to a line as well as a stream carried in that line. Samereference numbers refer to similar components.

FIG. 1 schematically shows a process scheme enabling selective lowtemperature separation of heavy hydrocarbons (C₅ ⁺) in a gas plant withflexibility to recover/reject LPGs.

The process scheme (or apparatus) is generally indicated with referencenumber 1.

A partially condensed hydrocarbon feed stream 10 such as natural gas issupplied to a first gas/liquid separator 2 at a certain inlet pressureand inlet temperature. In the embodiment of FIG. 1 the feed stream 10 isfed as two different streams, viz. a higher feed stream 10 a and a lowerfeed stream 10 b. If desired the feed stream 10 may be split in morethan two sub-streams. The higher feed stream 10 a is pre-cooled in heatexchanger 6 and fed to the separator 2 at first inlet 11; the lower feedstream 10 b is fed to the separator 2 at second inlet 12. In the shownembodiment, stream 10 a is cooled against another stream in the process(i.e. stream 60). However, any other cooling may be used, if desired.

Typically, the feed stream 10 has a temperature in the range fromambient to 90° C., preferably from 20° C. to 80° C. Preferably thepressure of the feedstream is in the range from more than 50 bar to 100bar, more preferably from more than 55 bar to 90 bar, even morepreferably from more than 60 bar to 80 bar. The temperature and pressureof the streams 10 a and 10 b is chosen to optimise a gas/liquidseparation step in separator 2. If desired, the pressure of the streams10 a and 10 b may have been adjusted in valves 13 and 14, respectively.

As mentioned above, stream 10 is fed to the gas/liquid separator 2 asstreams 10 a and 10 b. There, the feed stream 10 is separated into afirst vaporous (i.c. overhead) stream 20 and a first liquid (i.c.bottom) stream 70. The overhead stream 20 leaves the separator 2 atfirst outlet 15 and is enriched in methane (and usually also ethane)relative to the feed stream 10.

The bottom stream 70 leaves the separator 2 at second outlet 16 and isgenerally liquid; stream 70 may contain hydrocarbons that can beseparately processed to form liquefied petroleum gas (LPG) products.Usually, the bottom stream 70 is subjected to one or more fractionationsteps to collect various natural gas liquid products.

The overhead stream 20 is led to an expander 3, thereby at leastpartially condensing the stream 20, thereby obtaining stream 30.Subsequently, stream 30 is fed to a second gas/liquid separator 4 atinlet 21. In the second separator 4, the partially condensed stream 30is separated into a second vaporous (i.c. overhead) stream 60 and asecond liquid (i.c. bottom) stream 40. The overhead stream 60 leaves theseparator 4 at outlet 22 and is generally vaporous; the bottom stream 40leaves the separator 4 at outlet 23 and is generally liquid.

Then the stream 40 is pressurized in pressurizing unit 5 to a pressureof at least 50 bar. The pressurizing unit 5 may be any suitable meansfor increasing the pressure such as a pump. The pressurized stream 50leaving the pressurizing unit 5 is subsequently returned to the firstgas/liquid separator 2, preferably at the warm (i.c. high) part thereof,at third inlet 17 of the first separator 2.

The first liquid stream 70 and the second vaporous stream 60 may be usedas product streams or may be further processed, if desired.

In the embodiment as shown in FIG. 1, the second vaporous stream 60 isused to cool the higher feed stream 10 a in heat exchanger 6.

Furthermore, the first liquid stream 70 is (after being optionallydepressurized in valve 33) fed (as stream 70 a) to a third gas/liquidseparator 7 (at inlet 34) thereby obtaining (at outlet 31) a thirdvaporous stream 80 and (at outlet 32) a third liquid stream 90.

The third vaporous stream 80 is combined with the second vaporous stream65 (i.e. stream 60 after being heat exchanged in heat exchanger 6) atjunction point 18 and is subsequently compressed in compressor 8 therebyobtaining product gas 100 which will usually be subjected to aliquefaction step in one or more heat exchangers (not shown) therebyobtaining liquefied natural gas (LNG). In case that stream 100 is to beliquefied, some further treatment steps may take place to remove anycontaminants that may solidify during the liquefaction process. As anexample a (n optionally additional) CO₂ removal step may take place.

Stream 80 may be compressed to about the same pressure of the secondvaporous stream 65 before stream 80 is combined with the second vaporousstream 65 at the junction point 18.

FIG. 2 schematically shows an alternative embodiment of the presentinvention to provide an integrated gas dew pointing and condensatestabilizing process, wherein the third column 7 is in the form of adebutanizer/stabilizer, thereby obtaining a third vaporous stream 80being enriched in butane and lower hydrocarbons (such as methane, ethaneand or propane) relative to the third liquid stream 90.

Furthermore, FIG. 2 shows that the third vaporous stream 80, beforebeing combined with stream 65 in junction point 18, has previously beencooled (as stream 80 a) against (an air cooler or water cooler or, asshown) an external refrigerant in heat exchanger 55, fed (as stream 80b) to a fourth gas/liquid separator 19 at inlet 41, and removed atoutlet 42 from the fourth gas/liquid separator 19 (as stream 80). Thefourth gas/liquid separator 19 functions as an overhead condenser drum.The liquid bottom stream 110 removed at outlet 43 from the fourthgas/liquid separator 19 is pressurized in pump 51 and returned as stream120 to the top (at inlet 33) of the debutanizer 7.

A part of the bottom stream 90 (or ‘condensate’) of thedebutanizer/stabilizer 7 is split off at splitter 56, heat exchanged asstream 130 against an external stream in heat exchanger 52 (functioningas a reboiler) and returned as stream 140 to the bottom (at inlet 35) ofthe debutanizer/stabilizer 7. The major part of the condensate stream 90is (after splitter 56) heat exchanged against the first liquid stream 70in heat exchanger 53 and subsequently against stream 10 b in heatexchanger 54 and used as a product stream.

In addition to or instead of heat exchanging stream 70 (or 70 a) againststream 90 (in heat exchanger 53), stream 70 (or 70 a) may be heatexchanged against stream 80 a, for example in heat exchanger 55.

If desired, one or more further gaseous and/or liquid streams (notshown) may be introduced into the debutanizer/stabilizer 7.

The line-up as used in FIG. 2 allows to produce a product gas stream 80with a surprisingly high content of LPGs (i.e. propane and/or butane)and a condensate stream 90 with a surprisingly high content of C₅ ⁺(i.e. pentane and higher components). As indicated above, stream 80 maybe used as a separate product stream, but will usually combined withstream 65 to enrich the latter stream.

Table I gives an overview of the estimated pressures and temperatures ofa stream at various parts in an example process of FIG. 2. Also the molefraction of methane is indicated. The feed stream in line 10 of FIG. 2comprised approximately the following composition: 75.2 mole % methane,9.2 mole % ethane, 4.3 mole % propane, 2.1 mole % butanes, 5.2 mole % C₅⁺, 1.2 mole % N₂ and 2.7 mole % CO₂. H₂S and H₂O were previouslyremoved.

TABLE I Methane Pressure Temperature [mole Line [bar] [° C.] fraction]10 67.7 61.1 0.752 20 66.8 −1.0 0.807 30 42.2 −21.3 0.807 40 42.2 −21.30.291 50 69.7 −19.2 0.291 60 42.2 −21.3 0.831 65 41.7 85.6 0.831 70 67.04.9 0.287 80 42.7 10.0 0.456 90 9.5 173.3 0.0 100 49.6 78.8 0.795 1108.9 10.0 0.027

The person skilled in the art will readily understand that manymodifications may be made, without departing from the scope of theappended claims.

As an example, the expander 3 and compressor 8 may be functionallycoupled.

1. A method of treating a hydrocarbon stream, the method at leastcomprising the steps of: (a) supplying a partially condensed feed streamto a first gas/liquid separator, the feed stream having a pressure >50bar; (b) separating the feed stream in the first gas/liquid separatorinto a first vaporous stream and a first liquid stream; (c) expandingthe first vaporous stream obtained in step (b), thereby obtaining an atleast partially condensed first vaporous stream; (d) supplying the atleast partially condensed first vaporous stream obtained in step (c) toa second gas/liquid separator; (e) separating the stream as supplied instep (d) in the second gas/liquid separator into a second vaporousstream and a second liquid stream; (f) increasing the pressure of thesecond liquid stream obtained in step (e) to a pressure of at least 50bar, thereby obtaining a pressurized second liquid stream; and (g)returning the pressurized second liquid stream obtained in step (f) tothe first gas/liquid separator.
 2. The method according to claim 1,wherein in step (a) the feed stream is supplied as at least twodifferent streams to the first gas/liquid separator, the feed streamcomprising a higher feed stream and a lower feed stream.
 3. The methodaccording to claim 2, wherein the higher feed stream is cooled before itis supplied to the first gas/liquid separator.
 4. The method accordingto claim 3, wherein the higher feed stream is cooled against the secondvaporous stream obtained in step (e).
 5. The method according to claim1, wherein the first liquid stream obtained in step (b) is supplied to athird gas/liquid separator thereby obtaining a third vaporous stream anda third liquid stream.
 6. The method according to claim 5, wherein thethird vaporous stream is combined with the second vaporous stream. 7.The method according to claim 5, wherein the third liquid stream is heatexchanged against the first liquid stream, before it is supplied to thethird gas/liquid separator.
 8. The method according to claim 5, whereinthe third liquid stream is heat exchanged against the lower feed stream.9. The method according to claim 5, wherein the third column is adebutanizer, thereby obtaining a third vaporous stream being enriched inbutane and lower hydrocarbons relative to the third liquid stream. 10.The method according to claim 1, wherein the second vaporous stream, isliquefied, thereby obtaining a liquefied hydrocarbon stream.
 11. Anapparatus for treating a hydrocarbon stream, the apparatus at leastcomprising: a first gas/liquid separator for separating a partiallycondensed feed stream O into a first vaporous stream and a first liquidstream; an expander for expanding the first vaporous stream; a secondgas/liquid separator for separating the expanded first vaporous streaminto a second vaporous stream and a second liquid stream; and apressurizing unit for increasing the pressure of the second liquidstream to a pressure of at least 50 bar before being returned to thefirst gas/liquid separator.
 12. The apparatus according to claim 11,wherein the first gas/liquid separator comprises at least two inlets forthe feed stream, including a first inlet for a higher feed stream and asecond inlet for a lower feed stream.
 13. The apparatus according toclaim 12, wherein the apparatus further comprises a heat exchanger forcooling the higher feed stream against the second vaporous stream. 14.The apparatus according to claim 11, wherein the apparatus furthercomprises a third gas/liquid separator for separating the first liquidstream into a third vaporous stream and a third liquid stream.
 15. Theapparatus according to claim 14, wherein the third vaporous stream canbe combined with the second vaporous stream.
 16. The method according toclaim 2, wherein the first liquid stream obtained in step (b) issupplied to a third gas/liquid separator thereby obtaining a thirdvaporous stream and a third liquid stream.
 17. The method according toclaim 3, wherein the first liquid stream obtained in step (b) issupplied to a third gas/liquid separator thereby obtaining a thirdvaporous stream and a third liquid stream.
 18. The method according toclaim 4, wherein the first liquid stream obtained in step (b) issupplied to a third gas/liquid separator thereby obtaining a thirdvaporous stream and a third liquid stream.
 19. The method according toclaim 6, wherein the third liquid stream is heat exchanged against thefirst liquid stream, before it is supplied to the third gas/liquidseparator.
 20. The method according to claim 6, wherein the third liquidstream is heat exchanged against the lower feed stream.